Bangong-Nujiang Suture Zone Research Articles

A review of the radiolarian faunas in the Bangong-Nujiang Suture Zone: Implications for the tectonic evolution of the Mesotethys Ocean

A review of the radiolarian faunas in the Bangong-Nujiang Suture Zone: Implications for the tectonic evolution of the Mesotethys Ocean

Induced subduction initiation near an ultra-slow spreading ridge: A case study from the Beila ophiolite in the north-central Tibetan Plateau

Subduction initiation of oceanic lithosphere is the key scientific problem of the plate tectonics on Earth. When, where, and how an oceanic subduction began remained obscure because of little geological records. Here, we report a newly documented ophiolitic sequence, including ultra-slow spreading ridge fragments (V1) and associated fore-arc lavas (V2) units in the Bangong-Nujiang suture zone, north-central Tibetan Plateau. The V1 units are controlled by detachment fault with peridotite, gabbro and pillow basalt. Peridotite exhibits uniform characteristics of abyssal peridotite, representing residues after low-degree melting of asthenospheric mantle. Mafic rocks display geochemical features of normal mid-ocean ridge basalt (N-MORB). These features, combined with zircon U-Pb dating, indicate that formation of the oceanic lithosphere occurred along an ultra-slow spreading ridge around 177–170 Ma in this area. The V2 units occur as a complete sequence from bottom to top of fore-arc basalt (FAB), basaltic andesite, boninite, and high-Mg arc-related rocks with formation ages of 160–150 Ma, which directly extruded and overlay on the V1 units. This sequence is identical to those of the Izu-Bonin-Mariana (IBM) fore-arc, and represent magmatic response to subduction initiation (SI). Thus, we propose that the Beila ophiolite preserved an entire process of oceanic transition from ultra-slow spreading oceanic ridge to the SI. Subduction initiation in the Beila area was induced by southward subduction transition after the closure of the northern branch of the Bangong-Nujiang Meso-Tethys Ocean and subsequent continental collision. This study provides direct geological evidence for delineating the geodynamic mechanism of an oceanic initial subduction.

Hotspot–subduction zone interactions and their resource effects at ∼120 Ma on the central Tibetan Plateau

The central Tibetan Plateau, particularly the Bangong–Nujiang suture zone, hosts abundant and complex Late Mesozoic geological records and world-class mineral resources. However, intense debate persists about the origin and tectonic setting of these geological records and mineral resources, which impedes a comprehensive understanding of regional Late Mesozoic tectonic evolution and mineralization. In this study, we investigated the petrochronology, geochemistry, and Sr-Nd-Hf isotopic characteristics of ∼120 Ma magmatic rocks in this region. We explored their petrogenesis and sources, and in conjunction with their spatiotemporal distribution and scale, we facilitated a first proposal that these magmatic rocks were derived from ∼120 Ma hotspot–subduction zone interaction and that world-class mineral deposits were formed in this geological setting. This study is the first to identify geological records of hotspot–subduction interaction in the central Tibetan Plateau and it has significant implications for gaining a deeper understanding of the closing processes of the Late Cretaceous Epoch in the Meso-Tethys Ocean. This study also highlights the substantial contribution of this hotspot–subduction zone interaction to the formation of world-class mineral deposits in the region, and thus has crucial significance for guiding mineral exploration in the central Tibetan Plateau.

Early Cretaceous A-Type Acidic Magmatic Belt in Northern Lhasa Block: Implications for the Evolution of the Bangong–Nujiang Ocean Lithosphere

A-type granites have been the subject of considerable interest due to their distinct anorogenic geological background. The A-type and arc-related granites are crucial in deciphering the evolution of the ocean closure and continental collision in the Tibet Plateau. The demise of the Bangong–Nujiang suture zone (BNSZ) and the Yarlung–Tsangpo suture zone was accompanied by the emplacement of volumes of syn-collisional and post-collisional granites. Controversy has persisted regarding the contribution of the collisional granites within the Lhasa Block to the growth of the Tibetan Plateau. This study provides key evidence about the evolution of the Lhasa Block and Bangong–Nujiang Ocean (BNO) by the newly documented 1200 km long, Early Cretaceous A-type acidic magmatic belt. The resolution was achieved through the utilization of petrology, whole-rock geochemistry, zircon U-Pb geochronology, and in situ zircon Hf isotope analysis of the Burshulaling Granites in the eastern segment and previous existing data in the central and western segment of the Lhasa Block. The Burshulaling Granites are characterized as peraluminous, high-K calc-alkaline series, indicating a post-collision setting with high temperature and low pressure. The zircon grains from two granite samples yield 206Pb/238U ages of 115–113 Ma. In situ zircon Hf analyses with 206Pb/238U ages give εHf(t) of −6.2–0.6, showing prominent characteristics of crust-mantle interaction. Granites from east to west exhibit whole-rock geochemical and geochronological similarities that fall within the well-constrained Early Cretaceous time frame (117–103 Ma) and track post-collisional A-type acidic magmatic belt along BNSZ. We argue that this magmatism resulted from slab break-off or orogenic root detachment, leading to melting and mixing of the lower crust. Meanwhile, this study indicates the existence of the Bangong–Nujiang Ocean southward subduction or a collapse following an Andean-type orogen.

Hydrochemical evolution and source mechanisms governing the unusual lithium and boron enrichment in salt lakes of northern Tibet

The large-scale salt lakes widely distributed in the Tibetan Plateau provide unique and potential resources for lithium (Li) and boron (B). The concentration and characteristics of elements in these salt lakes resemble those found in geothermal water in northern Tibet, which highlights both as crucial sources of rare elements. This study presents comprehensive analyses of the hydrochemical composition and isotopes of B, strontium (Sr), hydrogen (H), and oxygen (O) in typical salt lakes, along with samples from surrounding springs and rivers in the Bangong-Nujiang suture zone of northern Tibet. The results reveal an extremely negative and anomalous distribution pattern of B isotopes in Zabuye Salt Lake that is closely associated with geothermal water. The enrichment of these elements in other salt lakes in the region is attributed to concentration of evaporation and sediment adsorption. Given the very high elevation of the recharge for geothermal water, the infiltration of salt lakes obviously cannot feed geothermal springs. On the contrary, we correlate the unusual enrichment of Li and B and other resources in salt lakes to geothermal spring discharge. The ultimate origin of these elements lies in magmatic sources, with later water-rock interaction leading to significant enrichment of incompatible elements such as Li, rubidium (Rb), cesium (Cs), and B in the geothermal system. The geothermal springs directly or indirectly fed the salt lakes, and with further evaporation, they became super-scale brine deposits.

Reconstruction of the evolution of the Yurongguo ophiolite: Constraints on the southward intra-oceanic subduction of the Bangong-Nujiang Tethys Ocean

The evolution of the Meso-Tethys Ocean has received much research attention, but whether there was intra-oceanic and the starting time of subduction remains controversial. In this study, we present detailed petrological, U-Pb geochronology, geochemical, and Sr-Nd isotope investigations on Yurongguo ultramafic rocks from the western part of the Shiquanhe-Lagkor-Jiali suture zone, south of the Bangong-Nujiang suture zone, including harzburgites and websterites. The harzburgites are characterized by V-shaped REE patterns and significant Eu depletion, showing significant sediment influence. The websterites are classified into two groups based on their petrological and geochemical, but they all show distinctly Nb-Ta enrichment. Yurongguo ultramafic rocks have low 87Sr/86Sr(t) but high εNd(t) values (+4.84 to + 8.55), indicating that these rocks evolved from a depleted mantle without the influence of arc. The zircon U-Pb age group includes 640 ± 30 Ma, 805 ± 13 Ma, 1002 ± 17 Ma, and 1781 ± 40 Ma, similar to the Precambrian basement of the central Lhasa subterrane. The harzburgites are the remnant of ∼ 30 % partial melting of the depleted mantle and metasomatized by a small amount of sediment melt. The websterites I are a cumulation of depleted mantle melt followed by metasomatized with Nb-Ta-rich material. The websterites II are the result of the reaction between the websterites I and sediment melt. Combined with previous studies, we proposed that these rocks may have formed during initial intra-oceanic subduction, which are the product of the early Jurassic subduction of the Meso-Tethys Ocean. The ancient continental basement of central Lhasa subterrane impacted the Early Jurassic igneous rock formed in the Bangong-Nujiang Tethys Ocean.

Spatial heterogeneity of Cenozoic provenance in the Nima and Lunpola basins of China: Implications for the origin of the low-relief topography in central Tibet Plateau

The central Tibet Plateau is characterised by low-relief topography of high elevation; however, the origin of this surface flatness is uncertain because of a lack of reliable evidence from palaeogeography and palaeodrainage. In this study, we conduct sedimentological and provenance analysis of the Nima Basin, one of several terrestrial basins developed along the Bangong–Nujiang suture zone since the middle Eocene. Our data show that the Dingqinghu Formation in the Nima Basin was deposited in fan delta, braided river and lacustrine environments in the late Eocene after 37 Ma. It received sediment consisting mainly of recycled detritus, including orbitolinid-bearing limestone with volcanic fragments from the Lhasa Terrane in the south. Comparison of the stratigraphic sections in the Nima Basin with those of the adjacent Lunpola Basin suggests that these regions are characterised by local but similar provenance signals. We suggest that a source-to-sink system once existed from the shallower Nima Basin eastward to the deeper and bigger Lunpola Basin. We hypothesise that the Oligocene–early Miocene source-to-sink system from the Nima to Lunpola may have played a role in the surface flattening of the central Tibet Plateau.

The Cretaceous suturing process of the eastern Bangong–Nujiang Ocean, Basu area

The Bangong–Nujiang Ocean played an important role in the formation of the Tibetan Plateau prior to the Cenozoic India–Eurasia collision. However, there are still uncertainties about the subduction polarity and timing of the Lhasa–Qiangtang collision. We conducted sandstone petrologic and detrital zircon U–Pb–Hf isotopic analyses on the Cretaceous Wada mélange, trench-fill strata and Duoni Formation in the Basu area in the eastern Bangong–Nujiang Suture Zone. The Wada mélange (∼114 Ma) exhibits block-within-matrix features and a detrital zircon U-Pb age spectrum characterized by multiple peaks at 114–180 Ma, 200–300 Ma, and 1800–2000 Ma. The trench-fill strata consist of coherent chert, sandstone, and mudstone, with a detrital zircon age spectrum dominated by a single peak at ∼120 Ma. The εHf(t) values of the 110–300 Ma detrital zircon grains in the Wada mélange and trench-fill strata range from −20 to +10 and are consistent with those of the South Qiangtang Terrane. The εHf(t) values of the ∼120 Ma detrital zircons are all negative, and these grains were likely derived from a remnant Cretaceous arc in the South Qiangtang Terrane to the north. These data suggest that the accretionary wedge was derived from the South Qiangtang Terrane during the northward subduction of the Bangong–Nujiang Ocean. The Duoni Formation (∼113 Ma) represents peripheral foreland basin deposits with the Lhasa Terrane as the basement. Provenance analysis indicates that these deposits received clastic material mainly from the South Qiangtang Terrane and to a lesser degree from the North Lhasa Terrane. Our results suggest that the Lhasa–Qiangtang collision occurred in the Early Cretaceous (∼113 Ma) in the eastern segment of the Bangong–Nujiang Suture Zone.

Provenance Analysis in the Nima Basin during Paleogene and Its Implications for the Decline of the Tibetan Central Valley.

It is unclear what caused the Bangong Nujiang suture zone in the central Tibetan plateau to rise from less than 2 km in early Cenozoic to more than 4 km at present. The zircon U-Pb ages and trace elements of samples from the Niubao Formation in the Paleogene of the Nima basin were analyzed and tested. Combined with the isostasy theory, the surface uplift height of the Nima Basin during the Cenozoic period was calculated. The zircon U-Pb age results of the Niubao formation are consistent with the ages of the Lhasa terrane on the south side of the basin, the Qiangtang terrane on the north side, and the uplift in central. The zircon Eu/Eu* results show that the crust in central part of Tibetan plateau thickened by ∼20 km in Paleogene, resulting in ∼3 km surface uplift. Sediments created a total of about 1 km of surface uplift throughout the Paleogene, and the deposition rate began to slow down significantly at ∼40 Ma. Therefore, it is inferred that in the early Cenozoic, the uplift of the valley was mainly caused by sedimentation. With the continuous downward subduction of the Indian plate, at about 40 Ma, factors such as crustal shortening dominated the uplift of the central valley, and the uplift caused by deposition only accounted for a very small part. In general, the uplift of the Central Valley in the Paleogene was mainly affected by crustal shortening, but a quarter of the surface uplift was caused by the accumulation of sediments.

Middle–Late Jurassic subduction erosion caused by intra-oceanic arc subduction in central Tibet

Abstract Subduction erosion is crucial in crustal material recycling. However, subduction erosion caused by intra-oceanic arc subduction has not been sufficiently investigated. In this study, we provide new geological, geochronological, geochemical, and isotopic data from Dongco granodiorites in the central Bangong–Nujiang suture zone of central Tibet to explore subduction erosion caused by intra-oceanic arc subduction. Analysis shows that the ca. 158–155 Ma Dongco granodiorites originated from the subducted oceanic plate, and they were contaminated with accretionary wedge when they intruded the Dongco ophiolite. This suggests that the Dongco ophiolite was emplaced in the subducted accretionary wedge before the Late Jurassic. Based on the intra-oceanic arc affinity and lack of volcanic rocks of Middle Jurassic Dongco ophiolite and other regional data, we believe that the main body of the central intra-oceanic arcs and a portion of the western intra-oceanic arcs in the Meso-Tethys Ocean subducted beneath the southern Qiangtang terrane during the Middle–Late Jurassic. In addition, the different degree absence of the Jurassic accretionary wedge, forearc region, and arc magmatic rocks in the southern Qiangtang terrane indicate that the central and western parts of the southern Qiangtang terrane experienced both vigorous and relatively weak subduction erosion during the Middle–Late Jurassic, respectively. Thus, there is a significant spatiotemporal coupling between subduction erosion of the southern Qiangtang terrane and intra-oceanic arc subduction. Based on these studies and the research on subduction erosion, we suggest that subduction of the main body of the central intra-oceanic arcs and partial subduction of the western intra-oceanic arcs in the Meso-Tethys Ocean caused both vigorous and relatively weak subduction erosion of the southern Qiangtang terrane during the Middle–Late Jurassic, respectively. In addition, the increase in subduction rate also promoted Middle–Late Jurassic subduction erosion of the southern Qiangtang terrane.

Polyphase Exhumation of the East Kunlun Orogenic Belt: Evidence from Modern River Detrital Zircon and Apatite Fission Track Dating

Abstract The East Kunlun Orogenic Belt (E-KOB) stands out as one of the most prominent basin-mountain geomorphic features in the northern interior of the Tibetan Plateau. It records a series of accretion-collision events from the Mesozoic to the Cenozoic. In particular, with the uplifting of the Tibetan Plateau, the E-KOB experienced intracontinental deformation and exhumation in the Cenozoic. Clarifying the exhumation history of the E-KOB is crucial to define the growth time and mechanism of the Tibetan Plateau. In this study, we apply detrital zircon fission-track (ZFT) and apatite fission-track (AFT) analyses on modern river sands in order to constrain the regional exhumation history of the eastern E-KOB. Four peak ages have been identified and interpreted as results of rapid exhumation correlated with intracontinental deformation. Two older peak ages at 144.7–141.0 and 114.6–82.1 Ma are in good accordance with the collision time of the north-south Lhasa-Qiangtang Block along the Bangong-Nujiang suture zone and the subsequent progressive deformation stage toward the north. Peak age at 60.9–45.3 Ma is coeval with the initial timing of the India-Asia collision. The youngest peak age at 25.1–18.3 Ma matches well with the extensive outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene time. The Cretaceous and early Cenozoic rapid exhumations suggest that the E-KOB has been involved in the intracontinental deformation induced by collisions of the Lhasa-Qiangtang and India-Asia from the south. It implies that the northern Tibetan Plateau likely has been elevated or was a structural high before the Eocene. In addition, some of our detrital samples show a younger ZFT peak age than the AFT peak age. We attributed this data bias to the contribution of hydrodynamic sorting and/or lithological difference. The combination of ZFT and AFT dating has advantages in eliminating interfering age signals in detrital thermochronology.

Late Mesozoic subduction-accretion in the southern Qiangtang: Insights from the Sumxi igneous complex of west-central Tibet

Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110−106 Ma igneous complex comprising basalts−basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts−basaltic andesites have SiO2 contents of 52.5−58.7 wt% and MgO contents of 2.89−4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [εNd(t) = −7.40 to −6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6−65.2 wt%), high Mg number (40−60), and more enriched εNd(t) values (−8.37 to −7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14−1032.70 ppm, Y = 14.86−15.37 ppm, Sr/Y = 54−68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide convincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma).

Lower Cisuralian brachiopod faunas from the Lhasa and South Qiangtang blocks in Tibet and their biostratigraphical and palaeobiogeographical implications

The opening time of the Bangong-Nujiang Ocean (BNO, also known as the Meso-Tethys Ocean) remains controversial. It was documented to have opened during the Permian, Early Palaeozoic or Mesozoic times. The Permian faunas and their palaeobiogeographical affinities of the Lhasa and South Qiangtang blocks bordered by the Bangong-Nujiang Suture Zone are crucial for reconstructing the tectonic evolution of both blocks and determining the opening time of the BNO. This paper describes two new lower Cisuralian brachiopod faunas from the uppermost Yunzhug Formation of the Lhasa Block and the Zhanjin Formation of the South Qiangtang Block. These two faunas exhibit a high similarity in composition, both containing species of Bandoproductus, Spirelytha and Sulciplica thailandica. Comparable brachiopod faunas are found from the glaciomarine diamictites in the blocks of the eastern Cimmerian belt including the Baoshan, Irrawaddy and Sibuma, as well as the Tethys Himalaya and the Gondwanaland. They are herein named the Bandoproductus-Spirelytha association, which exhibits apparent Gondwanan affinities. Cluster analysis and principal components analysis are employed to conduct a palaeobiogeographical reconstruction of the Asselian–Sakmarian brachiopod faunas primarily from the aforementioned tectonic units. A single palaeobiogeographical unit, the Indoralian Province, is recognised and represented by the unified brachiopod association. This suggests that most of the Cimmerian blocks, including the Lhasa and South Qiangtang blocks, were attached to the northern Gondwanan margin during the early Cisuralian. This is in contrast to our previous results revealed by multiple quantitative analyses for the Kungurian–Roadian brachiopod faunas in the Tethys, which show distinct faunal differences between the Lhasa and South Qiangtang blocks. This palaeobiogeographical discrepancy suggests that the two blocks probably had varying northward drifting tempos after they detached from the Gondwanaland, that is, the South Qiangtang Block drifted faster and incorporated into the temperate-warm region of the Northern Cimmerian Subprovince; whereas the Lhasa Block drifted relatively slowly and retained to be close to the northern Gondwanan margin and belonged to the Southern Cimmerian Subprovince during the late Cisuralian. The faunal differences provided critical implications that the BNO probably had opened in the Artinskian and reached a certain width with distinct faunal differences during the Kungurian.

Fault slip behavior and segmentation revealed by geomorphic offset clusters: An example from the middle Riganpei Co fault, central Tibet

High-resolution topographic datasets are useful for revealing offset features of fault geomorphology and are thus important for reconstructing the evolution history of faults. The Riganpei Co fault, located in the central Tibetan Plateau, is an important active fault on the northern boundary of the Bangong-Nujiang suture zone. In this study, WorldView-2 stereo images were used to generate high-resolution topographic data in a 1 km range on both flanks of the middle section of the Riganpei Co fault. We interpreted the tectonic geomorphology along the fault, classified it into distinct units, and measured the horizontal displacement of each unit. Within approximately 70 km along the fault, we obtained 396 displacement measurements ranging from 0 to 60 m. Given the evident uneven distribution of displacement, the fault was divided into four segments. In each fault segment, the reconstructed slip distributions revealed a tendency of larger displacements in the center and smaller displacements toward the ends. The cumulative offset probability distribution (COPD) of displacements displays 4–5 offset clusters in the range of 0–30 m for each segment, suggesting that at least four large earthquakes ruptured this fault. The binned COPD demonstrated that there are differences in the displacement accumulation modes among these fault segments, where the middle segment follows a characteristic slip accumulation mode. Reconstructed cumulative slip profiles revealed a complex pattern of strong earthquake activities along the Riganpei Co fault.

Petrogenesis of Early Cretaceous andesites and mafic dikes in central Tibet: Implications for the growth of continental crust in collision zones

Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock εNd(t) (−6.5 to − 6.1) and zircon εHf(t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative εNd(t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon εHf(t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone.

Paleogene sediment provenance in the Gaize Basin: Implications for early Cenozoic paleogeography of central Tibet

A comprehensive study of the sediment source-to-sink history of basins arranged along the Bangong-Nujiang Suture Zone (BNSZ) is critical for understanding the early Cenozoic topography and drainage systems of central Tibet and its implications for the uplift history of the Tibetan Plateau. Nevertheless, previous provenance analyses work mainly focused on the middle-eastern segment of the BNSZ, those Paleogene basins in the western segment have received less attention, which hinders a full understanding of the early Cenozoic topography of central Tibet. In this study, based on a chronostratigraphic framework developed for the lower Cenozoic continental redbeds of the Gaize Basin, we applied detrital zircon U-Pb geochronology to the well-exposed Paleogene Suonahu Formation in order to reconstruct the basin's source-to-sink history. Integrating these new data with detrital zircon U-Pb results from the underlying Kangtuo Formation, we recognize a major secular shift in sediment provenance characterized by inputs of Lhasa-derived clasts during the Late Eocene. Moreover, we applied statistical analysis to compare the U-Pb age spectra of Eocene strata in the Gaize-Nima-Lunpola basins in order to assess similarities between them and potential source regions. Our study confirms a paleogeographic scenario in which these Paleogene basins most likely were part of a low-elevation intermontane valley.

Coexisting High-Al and High-Cr Chromitites in the Dingqing Ophiolite (SE Tibet): Inferences to Compositional Heterogeneity in the Tethyan Upper Mantle

The Dingqing ophiolite represents a significant allochthonous ophiolite nappe in the eastern segment of the Bangong–Nujiang suture zone in southeastern Tibet. The microanalytical data of associated podiform chromitites classify them into two distinct varieties: high-Al and high-Cr. The coexistence of both high-Cr and high-Al chromitites in the Dingqing ophiolite suggests a complex or multistage evolutionary history of the host rocks. New petrological and geochemical analyses are used herein to unravel the interrelationships between the chromitite ores and host rocks and assess the mechanism of formation. The Dingqing ophiolitic nappe is made up mainly of harzburgite, dunite, and less abundant pyroxenite and gabbro. Several small lens-shaped bodies of chromitite ore are mostly confined to the harzburgite rocks, with ore textures varying from massive to sparsely disseminated chromite. In addition to magnesiochromite, the orebodies contain minor amounts of olivine, amphibole, and serpentine. The textural relationships provide compelling evidence of plastic deformation and partial melting of the associated peridotites. Detailed examination of the Cr-spinel grains reveals a wide range of composition, spanning from high-Al (Cr# = 3.18–59.5) to high-Cr (Cr# 60.3–87.32). The abundances of the platinum-group element (PGE) in chromitites are significantly variable (93 to 274 ppb). Formation of the Dingqing peridotites most likely took place in a mid-ocean ridge (MOR) setting, and subsequent modifications by supra-subduction zone (SSZ) melts resulted in heterogenous or mixed geochemical characteristics of these rocks. Chemistry of the spinel–olivine–clinopyroxene assemblage demonstrates multiple stages of partial melting of the source mantle rocks, including an early phase of restricted partial melting (~20%–30%) and a later phase of extensive partial melting (>40%). The formation of the high-Al chromitite type was associated with the early phase (constrained melting), whereas extensive partial melting in the late stages likely led to the accumulation of high-Cr podiform chromitite bodies.

Mesozoic felsic magmatism along the Bangong–Nujiang suture zone, Tibet: Temporospatial variations and tectonic implications

The subduction history and closure of the Meso-Tethys Ocean have not yet been well constrained using data from felsic rocks along the Bangong–Nujiang suture zone. Here we present geochronological, geochemical, and isotopic data from Early Jurassic–Late Cretaceous felsic rocks in the Laguo Co area of central Tibet. We combine our results with published data from coeval felsic rocks to explore the temporospatial variations in magmatism and the implications for subduction in and closure of the Meso-Tethys Ocean. The studied rocks include 182–180 Ma granodiorite, 109 Ma granite porphyry, and 68 Ma rhyolite, and zircon grains from each yield different εHf(t) values (+8.9 to +14.0, −5.5 to +0.7, and −4.9 to +1.1, respectively). The rocks have low-K tholeiitic to medium-K calc-alkaline compositions, low Y and Yb contents, and positive Sr anomalies. These data suggest an oceanic source for the Early Jurassic granodiorite and a crustal source for the Cretaceous granite porphyry and rhyolite, as well as a tectonic transition from an oceanic arc during the Early Jurassic to continental collision during the Cretaceous. The temporospatial variation in felsic magmatism corresponds to intra-oceanic subduction, bidirectional subduction, oceanic ridge or flat subduction, slab rollback, slab breakoff or sinking, and lithospheric delamination. The timing of closure of the Meso-Tethys Ocean is constrained to the Jurassic–Cretaceous, as evidenced by the major differences of the magma sources and formation processes between the Jurassic and Cretaceous felsic rocks. Finally, we present a model showing that oceanic closure occurred earlier in the east and north and later in the west and south.

Early Jurassic lithospheric delamination in the Amdo microcontinent, central Tibet: Inferred from coeval OIB- and MORB-like gabbros

The Bangong–Nujiang suture zone (BNSZ) is the remnants of the Meso-Tethyan main oceanic basin in the Tibetan plateau. It is noted that several microcontinents, e.g., Amdo and Jiayuqiao, occur within the Meso-Tethyan ocean basin. These microcontinents were strongly reworked associated with the subduction and closure of this ancient ocean, thus they play a key role in understanding the evolution of this ocean. In this contribution, we report newly documented Early Jurassic gabbros (ca. 182–175 Ma) from the Amdo microcontinent in the middle segment of the BNSZ. The gabbros can be subdivided into two groups, i.e., OIB (ocean island basalt) and MORB (mid-ocean ridge basalt) groups. The OIB-group gabbros are characterized by highly enriched NdHf isotopic characteristics (zircon εHf(t) = −11.7 to −8.9; whole-rock εNd(t) = −11.0 to −10.1). They are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (> 85 km) and subsequent continental crustal inputting. The MORB-group gabbros belong to the tholeiitic series and have similar compositions to those of MORB. They were most likely derived from a depleted mantle source (zircon εHf(t) = +3.7 to +16.5; whole-rock εNd(t) = +4.9 to +8.1) in a relatively shallow depth (< 85 km). Integrating previous studies with the data presented in this contribution, we suggest that the generation of coeval OIB- and MORB-like gabbros are related to lithospheric delamination after the Qiangtang-Amdo collision and the closure of the Bangong-Nujiang ocean basin. This result will give new constraint on the evolution of the Meso-Tethyan Ocean.

Divergent double subduction of Bangong-Nujiang Ocean revealed by high-resolution magnetotelluric data at 86°E in the northern Tibetan Plateau

The Tibetan Plateau was mainly produced by the assembly of several terranes resulting from ocean closure and continental collision. The formation and evolution of the Bangong-Nujiang Ocean (BNO) played a prominent role in the Tibetan Plateau accretion before the Cenozoic collision between the Indian and Eurasian continents. The closure mode of the BNO between the Lhasa and Qiangtang terranes has been controversial, involving subduction polarity, i.e., regarding whether there was only northward subduction beneath the Qiangtang terrane or both southward and northward subduction beneath the Lhasa and Qiangtang terranes. To resolve this issue, a dense broadband magnetotelluric (MT) profile transecting the Bangong-Nujiang suture zone (BNS) at 86°E was completed. After standardized data processing and analysis, geological interpretation can be performed on the two-dimensional (2-D) inversion results of the MT data. Through a large number of inversion experiments, we obtained a reliable lithospheric electrical structure beneath the BNS and its adjacent regions. Our results show that the upper crust in our study area is featured by high resistivity overall, but it is cut by several fault zones, indicating that it may be influenced by the late strike-slip fault caused by the India-Asia collision. The middle-lower crust of the northern Lhasa terrane is highly conductive and may represent juvenile crust. The existence of a northward and southward inclined electrical gradient zone in the BNS may indicate that the BNO was closed by north-south bidirectional subduction.